Burkitt lymphoma, a tumor of B lymphocyte, immunoglobulin producing, cells is associated with specific chromosomal breakpoints in the precise regions where the immunoglobulin gene loci, kappa, lambda, and heavy chain reside. Our analyses of the Burkitt lymphoma system over the past few years led us to consider the possibility that what we were observing in this tumor vis a vis its specific trnaslocations might be generalized to the issue of all chromosomal translocations. Given the involvement of the immunoglobulin gene encoding regions in the translocations associated with this immunoglobulin producing tumor we asked whether translocations seen in cells of other differentiated function frequently involved the regions to which genes responsible for that differentiated function resided. An obvious choice to start this study were globin producing erythroleukemias because of the general availability of globin DNA probes to use for the analysis, and because production of globin was such a clearly differentiated function of these cells. In collaboration with cytogeneticists and hematopathologists at the Medical College of Virginia we have karyotyped two patients with newly diagnosed erythroleukemia and found a number of karyotypic abnormalities including chromosomal aberrations of the globin encoding regions (patient 1, t(7; 11); (patient 2, t(16;17)) in the tumor cells but not normal fibroblasts from both these patients. Our hypothesis is not that erythroleukemia is necessarily caused by a translocation into the globin encoding regions. We feel, however, that in cells that have activated or are in the process of activating their globin loci, the chromosomal regions to which these loci map will have an increased susceptibility to undergo karyotypic aberration compared to quiescent areas of the genome.